1. Technical Field
This invention relates to a method of establishing a path through an optical communications network.
2. Related Art
In an optical network, data is transmitted over optical fibres that interconnect network nodes (commonly called routers or switches). As data traverses the network it will traverse one or more network nodes. At each network node data is received on an input port, converted from an optical signal to an electrical signal, processed in the electrical domain, converted from an electrical to an optical signal and transmitted on the relevant output port. This conversion from an optical signal to an electrical signal and back again is called Optical-Electrical-Optical (OEO) conversion.
There are two main consequences of having OEO conversion at every node. Firstly, OEO conversion re-shapes, re-times and re-transmits the optical signal at every node meaning that each link in an end-to-end path can be considered in isolation and any fault or impairment on one link does not have a knock-on affect on subsequent links. Secondly, OEO conversion enables a node to receive an optical signal on a given wavelength and re-transmit it on a different wavelength, i.e. the nodes can perform wavelength conversion. However, OEO conversion is expensive to implement and acts as a bottleneck in the network because the maximum speed at which data can be switched from an input port to an output port depends on how fast the underlying electronics in each network node can operate.
In an all-optical network, data is also transmitted over optical fibres that interconnect network nodes. However at each network node, data is received on an input port, switched in the optical domain and transmitted on the relevant output port without any OEO conversion taking place. The lack of OEO conversion in all optical networking nodes (more commonly called optical cross connects (OXCs)) results in the cost of networking nodes being reduced. It is usual in an all optical network for several different wavelengths to be used simultaneously on each optical fibre by utilising wavelength division multiplexing (WDM).
However, the lack of OEO conversion in an all-optical node means that nodes can no longer perform wavelength conversion so it is necessary to use the same wavelength along the entire end-to-end path. This is known as the wavelength continuity constraint and can result in a situation where an end-to-end path is not usable because despite the network having spare capacity (i.e. wavelengths that are not being used to transmit data), there is no single common wavelength available on all the links on the end-to-end path.
When bound by the wavelength continuity constraint, it is not possible to consider each link on the end-to-end path in isolation since any fault or impairment at one OXC or on any one link will have an accumulative affect along the path. Instead, it is necessary to look at the end-to-end optical loss (the loss associated with switching a light of a given wavelength inside an OXC) across the entire end-to-end path in order to determine whether or not a path is usable.
Many current path selection algorithms start out with the assumption that all end-to-end paths are usable. As a result, it is not until an attempt is made to transmit data along the path that it is discovered that the selected path is not usable. This adds to the operating costs of the network and may even involve manual intervention in an automatic process.
US patent application US 2003/0011844 describes how the viability of an end-to-end path can be checked once the complete path end-to-end path has been selected. The disadvantage of checking the viability of an end-to-end path in this way is that additional (and hence wasted) processing may be expended in selecting an end-to-end path that turns out not to be viable. Consequently, the operating costs of the network would increase.